1. Field of the Invention
The invention relates to the use of certain chemical elements for inhibiting the formation of precipitates comprising zirconium molybdate.
This invention most particularly finds application in the field of reprocessing of used fuels, where it may be used for inhibiting the formation of precipitates comprising zirconium molybdate in nitric reprocessing solutions comprising the element molybdenum and the element zirconium.
2. Description of the Related Art
Molybdenum and zirconium are two major fission products encountered during methods for reprocessing used fuels, notably during steps for dissolving irradiated fuels and for concentrating fission products and for storing the resulting solutions. Beyond a concentration above that of the solubility of the elements molybdenum and zirconium in the liquid medium in which they are located (generally a nitric solution), the aqueous forms of molybdenum and zirconium may precipitate in order to form solid precipitates comprising zirconium molybdate hydrate.
The formation of these precipitates may mainly pose two types of problem.
The first is the formation of fouling layers which may hamper proper operation of the apparatus set into play in the reprocessing of used fuels, or even damage them irreversibly.
The second one is the carrying off and accumulation of secondary ions in the precipitates, which may occur either by inclusion in the latter or by adsorption at the surface of the latter. For example, plutonium is carried off by formation of mixed zirconium and plutonium molybdate precipitate. Another example is cesium, which may be carried off by its adsorption on zirconium molybdate. Thus, generally, accumulation of radio-elements and of fissile material may result therefrom and this in any apparatus of the process in fine receiving the dissolution solutions or receiving zirconium molybdate formed upstream.
In order to find a remedy for these drawbacks, curative actions have been undertaken in order to remove the formed zirconium molybdate precipitates. These curative actions may be of a mechanical order or a chemical order, notably by dissolving the precipitates formed by action of soda or of a solution comprising carbonate ions. But these solutions have consequent impacts, since they generate additional effluents, which should then be treated.
Presently, no technical suggestion intended for hindering the formation of precipitates comprising zirconium molybdate, notably in solutions for reprocessing used fuels, has been proposed in the literature. The only studies relating to zirconium molybdate in the method for reprocessing used fuels are focused on the formation kinetics of precipitate depending on the operating conditions, on temperature, acidity and concentration. Other more general studies have described possible applications of zirconium molybdate as a catalyst, ion exchanger, intermediate product for the formation of ZrMo2O8.
Therefore, there exists a real need for a technical solution with which it is possible to reduce, or even prevent the formation of precipitates comprising zirconium molybdate in an aqueous solution, such as a nitric solution, comprising the element molybdenum and the element zirconium, other than by acting on the operating conditions of temperature, acidity and concentration.